1. Field of the Invention
The present invention relates to a method for optimizing the operation of a simulated moving bed system for separating constituents.
The present invention more specifically relates to a method for determining directly an optimum initialization point very close to the working point to be reached in a simulated moving bed process for separating constituents.
The method notably applies to the separation of aromatic hydrocarbons or optical isomers.
2. Description of the Prior Art
In industry, there are many continuous separation processes based on selective adsorption of at least one component among several in a mixture of fluids, notably processes known as simulated countercurrent chromatography processes where the property of certain porous solids, in the presence of liquid, gaseous or supercritical mixtures, of retaining more or less significantly the various constituents of the mixture is used.
Separation or fractionation processes based on chromatography are most often implemented in a device comprising a set of n chromatographic columns or column fractions interconnected in series (generally 4xe2x89xa6nxe2x89xa624), forming an open or closed loop. A porous solid of determined grain size, distributed in different beds, constitutes the stationary phase.
Injection points for the mixture to be separated, comprising at least two constituents and the solvent or desorbent, and fluid extraction points are distributed along this loop and delimit most often four zones I, II, III, IV, the constituent preferentially sought being mainly either in the extract (Ex) or in the raffinate (R). Identical liquid flows run through all the columns or column fractions of a zone.
The raffinate flow rate is equal to the sum of the inlet flow rates minus the extract flow rate. In addition to these controlled flow rates is a recycle flow rate.
In a process known as true moving bed (LMV) process, a stationary concentration profile develops in the separation loop where the position of the points of injection of a feedstock F, of an eluent El, and of draw-off of an extract Ex and of a raffinate R remains fixed. The adsorbent solid and the liquid circulate in a countercurrent flow. A solid carrying system and a recycling pump P, both placed in the loop (at the junction of zones I and IV where the only species present in the liquid as well as in the solid is the elution carrier fluid), allow respectively to drive the solid from the bottom to the top and conversely the liquid from the top to the bottom.
Processes known as simulated moving bed (LMS) processes avoid a major difficulty inherent in true moving bed processes, which consists in correctly circulating the solid phase without creating attrition and without considerably increasing the bed porosity in relation to that of a fixed bed. In order to simulate the displacement thereof, the solid is placed in a certain number n of fixed beds placed in series and it is the concentration profile which is displaced at a substantially uniform speed all around an open or closed loop by shifting the injection and draw-off points.
In practice, successive shifting of the injection and draw-off points is performed by means of a rotary valve or more simply by means of a set of properly controlled on-off valves. This circular shift, performed at each period, of the various incoming-outgoing liquid flows in a given direction amounts to simulating a displacement of the solid adsorbent in the opposite direction.
The main inlet flow rates are the feedstock flow rate and the eluent flow rate. The outlet flow rates are the extract flow rate and the raffinate flow rate. At least one of these flows (raffinate, eluent, extract) is withdrawn or injected under pressure control. The raffinate flow rate is equal to the sum of the inlet flow rates minus the extract flow rate. In addition to these controlled flow rates there is a controlled recycle flow rate. The relative location of each of the four flows around the beds thus defines four distinct zones in the case of the process shown in FIG. 5.
Countercurrent or cocurrent simulated moving bed chromatography processes are for example described in U.S. Pat. Nos. 2,985,589 and 4,402,832.
U.S. Pat. Nos. 5,457,260 and 5,470,482 describe a process controlling a simulated moving bed system for separating a mixture of constituents, comprising two loop interconnected multiple-bed columns, where at least one characteristic such as the purity of a constituent or the yield thereof or a combination of both is controlled.
Determining the parameters necessary to the operation of a separation loop is difficult because many variables are involved in the process. For a given stationary phase and eluent, a composition of mixture to be purified and fixed constituents to be drawn off, the following values have to be calculated:
four liquid flow rates and a permutation period,
the concentration of the mixture to be separated,
the number of columns and the number of columns per zone,
the length and the diameter of the columns.
Charton F. and Nicoud R. M. (1995), in Journal of Chromatography, A 702, 97, 1995, also describe a method for calculating various characteristics of separation systems.
For a given supply concentration, these flow rates can be obtained empirically but the optimum solution lies in a limited zone of a five dimensional space (4 liquid flows and either the flow of solid in the case of a true moving bed, or the permutation period T in the case of a simulated moving bed), which can be reached most often only after a considerable time without being certain that the optimum point has been reached.
In order to find the optimum conditions for controlling or dimensioning a true (LMV) or simulated (LMS) moving bed separation system, it is preferable to find a model representative of the separation process taking account of adsorption phenomena, mass transfer and of the properties of the fluid flow through the porous solid phase, and to replace a burdensom empirical approach by simulations. This approach by simulation can however be just as burdensome if it is not properly performed. In order to implement it advantageously and to reduce the number of trial-and-error cycles necessary to stabilize operation, it is preferable to start from a well-targeted initial point.
Determining the adsorption isotherms describing the adsorption of the components to be separated is an essential stage for formation of the model. For a single-constituent system, the relation between the concentrations in the adsorbed phase {overscore (C)} and in the liquid phase C is sought, at equilibrium and at a given temperature. Even though this relation can be linear in a wide concentration range, it is generally non linear. For a multi-constituent system, a competition for access to the adsorption sites, which are in limited number, adds to the non linearity of the equilibrium isotherm. The adsorbed phase concentration {overscore (C)} of a compound i then depends on all the concentrations Ci in the liquid phase. A relation of the form: {overscore (C)}i=fi(C1,C2, etc) is therefore sought for each constituent.
Various initialization possibilities for the variables involved in the separation units are proposed in the literature according to the type of adsorption isotherms generated. According to well-known examples, these are linear isotherms or Langmuir type competitive isotherms. However, none of the known instances provides a satisfactory solution. The most realistic ones are the Langmuir competitive isotherms but they give selectivites independent of the composition whereas in practice it is observed that selectivity evolves with the concentration. It is therefore advisable to use isotherms describing more realistic adsorption effects for initialization of true moving bed (LMV) as well as of simulated moving bed (LMS) type separation units.
The method according to the invention allows, considering the separation results sought (flow rates, concentrations, purities, any isotherms), to direct determination of an initial working point very close to an optimum working point of a simulated moving bed (LMS) system for separating the constituents of a mixture containing an adsorbent solid phase, comprising a (closed or open) loop including several zones delimited by fluid injection and extraction points, by selecting initial values to be imposed on the fluid flow rates, knowing the flow rate of the feedstock injected in the loop and the concentrations of the various constituents of this feedstock, from flow rates corresponding to an equivalent true moving bed (LMV) loop.
The method comprises:
determining the flow rates of an equivalent loop by:
a) using a thermodynamic adsorption model that is not limited to a particular shape of adsorption isotherms,
b) locating, along the equivalent loop, compressive or dispersive fronts in concentrations of the various constituents,
c) determining propagation velocity of the fronts in reference to material balances, and
d) equating the propagation velocity of key compositions to zero at particular points of the loop, and
e) determining the corresponding flow rates of the simulated moving bed loop.
According to an embodiment applied to the separation of a mixture comprising at least two constituents A, B, where the solid phase adsorbs constituent B more than constituent A, in a loop comprising at least three zones, a zone I, a zone II and a zone III, with a liquid injection point at the inlet of zone I, an extract draw-off point between zone I and zone III, and a raffinate draw-off point at the outlet of zone III, concentrations of the feedstock and adsorption isotherms being known, a working point of the equivalent loop is determined by carrying out the following stages:
1) the flow rate in zone I is determined by equating the propagation velocity of constituent B to zero,
2) the flow rate in zone II and the concentration of constituent B in the extract are determined from an estimation of the concentration of constituent B in zone II and from the solution of the characteristic equation,
3) the respective concentrations of constituents A and B in zone III are determined by means of an iterative procedure including:
a) initializing the respective concentrations,
b) determining a flow rate in zone III related to the propagation velocity of compressive fronts, and
c) checking a material balance for constituent B;
4) the respective concentrations of constituents A and B are varied at the inlet of zone III while keeping the same characteristic in order to satisfy the global material balance for constituent B,
5) the concentration of constituent A in zone III is determined, and
6) the previous stages are repeated until the material balance of constituent A is checked.
According to another embodiment variant applied to a loop comprising four zones including a zone I, a zone II, a zone III and a zone IV, with a fluid injection point between zone IV and zone I, an extract draw-off point between zone I and zone II, an injection point for a feedstock including the mixture between zone II and zone III, and a raffinate draw-off point between zone III and zone IV, the concentrations of the feedstock and the adsorption isotherms being also known, the working point of the equivalent loop is determined by carrying out stages 1) to 6), with stage 5) additionally comprising determining a flow rate in zone IV, related to the propagation velocity of the compressive fronts.
Determining the initial working point is mainly obtained by equating the displacement velocity of the constituents to zero in order to obtain, as the case may be, a pure constituent in the extract or a pure constituent in the raffinate, or to obtain a defined intermediate purity in the extract or in the raffinate.
According to an embodiment, the equivalent loop control rates are determined by using favorable type isotherms (for example type I isotherms from the well-known Brunauer classification).
According to another embodiment, an inverse algorithm is applied for locating the compressive and dispersive fronts, with an unfavorable isotherm (of type III for example in the same classification).